The present invention relates to a video signal processor for a liquid crystal display (LCD), particularly a color LCD.
A display using liquid crystal (LC) is extensively used with various kinds of equipment. Especially, color LC is a recent achievement in the imaging art and is drawing much attention as a promissing substitute for the traditional cathode ray tube of a color television (TV). Actually, a portable miniature color TV and a wall-hanging type color TV implemented by color LC are available today. A color LCD has a plurality of color LC panels which are driven by a drive circuit. While the drive circuit drives the individual LC panels, a video signal processor processes an incoming composite video signal and delivers the resultant signal to the LCD to cause the latter to display it. Usually, the video signal processor has a Y/C separating circuit for separating a luminance signal (Y signal) and a color signal or chroma signal (C signal) from the composite video signal and outputting the Y and C signals. A decoder generates chrominance signals R-Y and B-Y in response to the Y and C signals. A matrix circuit is supplied with the Y signal and the chrominance signal R-Y and B-Y and in turn generate color signals R (red), G (green), and B (blue). The matrix circuit feeds each of the color signals R, G and B to a different clamping circuit. In response, each clamping circuit clamps the blanking level or pedestal level of the associated color signal, R, G or B to one half of a predetermined peak-to-peak voltage level of a video signal which should be applied to the associated LC panel of particular color. A switching circuit is associated with each of the clamping circuits for switching the output signal of the latter to produce an inverted and a non-inverted signal alternately field by field, the inverted and non-inverted signals being applied to the associated LC panel. Such switching circuits serve to extend the service life of the LC panels.
The prior art video signal processor has a problem left unsolved, as follows. When the inverted and non-inverted signals to be applied to any of the LC panels include components which are lower than the pedestal level (e.g. undershoot components), a reproduced picture appears double to the viewer's eye at the edges thereof due to the inherent relationship between the color signal lever and the transmittance of the LC panel. Generally referred to as light-and-dark inversion, this kind of phenomenon degrades the picture quality to a critical extent. While such an occurrence may be coped with by changing the pedestal level with the undershoot components being taken into account, this brings about another problem that a picture cannot be reproduced with sufficient contrast.